1. Field of the Invention
The present invention relates to a roller and a fixing apparatus including the roller.
2. Description of the Related Art
As a fixing apparatus that is included in an electro-photographic printer or copier, a film heating type fixing apparatus is known, which is constituted by a plate heater, a fixing film that moves with contacting the heater, and a pressure roller with which the heater forms a nip portion via the fixing film.
In the film heating type fixing apparatus, if small-sized recording materials are continuously printed, a temperature rise in a non-paper pressing region, that is, an excessive rise of temperature in a region where film or recording material of the pressure roller does not pass (non-paper passing region), is easily generated. If the state of the excessive temperature rise in the non-paper passing region continues, the pressure roller or the like tends to be damaged.
Therefore, in Japanese Patent Application Laid-open No. 2009-31772, a pressure roller, having an elastic layer constituted by two elastic layers whose thermal conductivity values are concentrically different around the core of the roller, is disclosed as a means of decreasing the above-mentioned temperature rise in the non-paper passing region. In this pressure roller, the thermal conductivity of the pressure roller in the longitudinal direction is increased by dispersing thermo-conductive filler, which is oriented in the longitudinal direction, in the surface side elastic layer, whereby the temperature unevenness of the pressure roller in the longitudinal direction is decreased. Furthermore, the thermal conductivity of the lower side elastic layer is set to be lower than the thermal conductivity of the surface side elastic layer, whereby the outflow of heat to the core is controlled.
However, in some types of pressure rollers having two elastic layers like this, a tapered portion is created at the edge of the elastic layers. If such a pressure roller is installed in the film heating type fixing apparatus and continuous printing is performed, it is observed that a portion of the surface of the fixing film, where an area around the tapered portion of the pressure roller contacts, is subject to abrasion more so than the surface where an untapered area contacts. The abrasion generated on the surface of the fixing film is hereafter called “surface layer abrasion”.
FIG. 7A shows a result of measuring the change of hardness of the pressure roller in the longitudinal direction using a micro-hardness meter when the pressure roller is in contact with a region where surface layer abrasion is generated on the fixing film. As shown in FIG. 7A, an abnormal hardness region HA, where the hardness of the pressure roller in the region near the tapered portion abnormally increases as the tapered portion is approached, was observed. FIG. 7B shows a result of measuring the pressure distribution in a nip portion N of a fixing apparatus using this pressure roller, using a pressure sensitive paper. According to FIG. 7B, the pressure of the nip portion N partially increases in the portion of the abnormal hardness region HA.
If an area where the hardness of the elastic layer is high is generated in an area near the tapered portion, created at the edge of the elastic layer of the pressure roller, the surface layer abrasion worsens, which is a problem.